There are plenty of good reasons:
1. Being mono compatible means that it has good phase coherency across the recording. Means you'll have a much better sounding mix.
2. There are plenty of "stereo" sources that are effectively mono. Car radios, clock radios, iPod docks, etc...
3. Recordings that are not mono compatable don't sound "right" Usually, it is due to the phase issues referenced in #1.
Just learn to record and mix with a mono compatible technique and you'll always be set. There is no excuse not to.